THE EVOLUTIONARY SELECTION OF UNICELLULAR EUKARYOTES

The evolutionary selection of eukaryotic cells is not immediatly clear. From a physical standpoint, endosymbiotic associations would not appear to have many advantages; diffusion of nutrients would be reduced leading to lower growth rates and the the cell would have to coordinate genomes in at least two compartments. However, we have many examples of "incomplete" endosymbiotic associeations that allow experimental testing of selection pressures. In extant organisms these mostly are involving a heterotroph and an oxygenic photoautotroph. The fundamenal conclusion is that recycling of nutrients, especially nitrogen, is critical to the stabilty of the symbiotic association. In this lecture, I will examine how loss of fixed inorganic nitrogen in the Proterozoic ocean would have potentially been a strong driver of novel endosymbiotic associations between heterotrophic and autotrophic microbes, first under anoxic conditions and subsequently under oxic conditions. The latter would have led to a positive feedback, where the evolution of large unicellular algae would have accelerated burial of organic carbon and facilitated the rise of oxygen - ultimately leading to the Cambrian radiation of multicellular organisms.